Flow Measurement

Electromagnetic Flowmeter Working Principle, Types Applications

         An electromagnetic flowmeter (magmeter) is a device that measures the flow rate of a conductive liquid by detecting the voltage difference across the liquid as it flows through a magnetic field. The voltage difference, also known as the electromotive force (EMF), is generated by the movement of the conductive liquid through the magnetic field.

What is meant by Electromagnetic Flow meter?

An electromagnetic flowmeter (magmeter) is a device that measures the flow rate of a conductive liquid by detecting the voltage difference across the liquid as it flows through a magnetic field. The voltage difference, also known as the electromotive force (EMF), is generated by the movement of the conductive liquid through the magnetic field.

Electromagnetic Flowmeter

Principle of Electromagnetic Flow Meter Based on Faraday’s Law

         The electro-magnetic flowmeter works based upon Faraday’s Law. The law states that whenever a conductor cuts lines of magnetic field, an induced voltage is generated and the magnitude of this induced voltage is proportional to the rate at which these lines are cut and the voltage is perpendicular to the plane of conductor and the magnetic field. The direction of the induced voltage is given by the Fleming’s right hand rule.

         An electromagnetic flow meter contains electrodes that record electromotive force and an electromagnetic coil that creates a magnetic field.

Electromagnetic Flow Meter Formula

Faraday’s Formula: E is proportional to V x B x L

 Where: E= K x V x B x L

                  E – Voltage produced in a conductor

                  V – Velocity of the conductor

                  B – Magnetic field strength

                  D or L – Length of the conductor

                  K – Instrument constant

Schematic of Electromagnetic Flowmeter

Construction of Electromagnetic Flow Meter

         An electromagnetic flowmeter consists of a permanent magnet, Pipe/ Tube & electrodes.

         A permanent magnet or an electromagnet, which could either be alternating current (ac) or direct current (dc), wrapped around an insulating tube. For the purpose of picking up the induced voltage, two electrodes should be positioned so that they are perpendicular to the magnetic field. Proper operation requires both fluid flow in the pipe perpendicular to the plane of magnetic flux lines and the direction of induced voltage parallel to the line joining the electrodes. Schematic of electric magnetic flow meter is shown below.

Schematic arrangement of Electromagnetic Flow meter
Section showing electrodes and magnetic field

Working of Electromagnetic Flowmeter

         The fluid is carried via pipe is subjected to a transverse magnetic flux, with a diameter that is equal to the pipe’s diameter and a velocity that is comparable to the average fluid velocity, the fluid itself serves as the conductor.

A voltage that is proportionate to the fluid velocity is induced as the fluid moves in relation to the field. The induced voltage is observed by electrode and connected to external signal condition circuits which processes the output.

How to use Electromagnetic Flowmeters?

         Below mentioned are some of the basic steps for using an electromagnetic flowmeter

  • Install the flowmeter in a horizontal pipe that is at least 5 pipe diameters long and has a straight run of at least 3 pipe diameters on either side of the flowmeter.
  • Connect the flowmeter to a power source and a flow rate display or recording device.
  • Open the valve on the pipe to start the flow of liquid.
  • Calibrate the flowmeter by adjusting the display or recording device to match the known flow rate of the liquid.
  • Monitor the flow rate on the display or recording device.
  • To stop the flow of liquid, close the valve on the pipe.

It’s important to follow the manufacturer’s instructions for installation, calibration, and maintenance of the flowmeter. The accuracy of the flow measurement can be affected by factors such as the conductivity of the liquid, the temperature of the liquid, and the presence of air bubbles in the liquid.

Determining Factors of Electromagnetic Flowmeter 

The factors that determine fluid in flow meter are

  • Conductivity
  • Acid/Caustics
  • Velocity
  • Grease Bearing Liquids
  • Abrasive Slurries

Characteristics of Electromagnetic Flowmeter 

  • It has a great ability to save energy, making it particularly well suited for measuring liquid-solid two-phase flows such sewage, slurry, ore pulp, water-coal pulp, paper pulp, and so on.
  • Low installation demand since only the liner and electrode are in direct contact with the measured medium, which allows for good non-corrodible and wearability to measure a variety of acid and alkali chemical solutions.
  • Measurement accuracy is unaffected by changes in fluid density, viscosity, temperature, pressure, and conductivity; it can also detect positive and negative two-directional flow rate, which has made it possible to measure flow rate accurately.
  • The wattage usage of the instrument is under 5VA.

Advantages of Electromagnetic Flow Meter

  • No moving parts
  • Unobstructed flow path
  • Suitability for working with corrosive liquids
  • Minimum flow conditioning required
  • There is no problem with measurements of laminar and turbulent flows because the working principle is independent of fluid density and viscosity, responding simply to average velocity
  • Ideal for slurries
  • Very high accuracies

Limitations of Electromagnetic Flow Meter

  • Heavy coatings (insulating & shorting) can result in measurement errors
  • Physical pressure & temperature limits
  • A fluid’s conductivity will rise when salts are added to it.
  • The usage of the meter is restricted to fluids that are electrically conductive, and it is appropriate for installations that can only withstand a slight pressure drop.

Selection of material for Electromagnetic Flowmeter 

The construction of Electromagnetic Flow Meter we need to consider the material selection for Electrode & lining

Electrode Material – Ti (Titanium),Ta (Tantalum), Hb, Hc as they are resistive to corrosion.

Lining Material – Ptfe (Poly tetra fluoro ethylene), pfa (per fluoro alkoxy) as they have stability of plastics such as chemical, electrical, smoke, flame, etc.

Types of Electromagnetic flowmeter

The electromagnetic flowmeter is classified based on application.

  • Inline model electromagnetic flowmeter
  • Insertion model electromagnetic flowmeter
  • Low flow model electromagnetic flowmeter

Inline model electromagnetic flowmeter

         The inline or in-line flow meter is comprised of a flow body that is installed in a process line in a more permanent manner. The flow profile of the process gas can be optimized to the built-in flow conditioner that is included in the meter configuration.

Insertion model electromagnetic flowmeter

         The insertion model EMF determines how quickly conductive liquids move through pipelines with a big diameter. Conductive fluids are raw sewage, wastewater, clarified water.

Low flow model electromagnetic flowmeter

         The low flow model EMF is used for chemical injections with a modest flow rate. This type, which has no moving parts, is able to process fluids that contain particulate matter without clogging or jamming, hence reducing the amount of maintenance that is required. Because it does not contain any metallic components, the meter is resistant to corrosion and is suitable with a wide variety of substances. The precision is preserved by using conductive fluids with densities and viscosities that range from 20 micro Siemens and higher.

Types of Electromagnetic Flowmeter

Excitation in Electromagnetic flowmeter

DC magnet Coil Excitation

  • Frequency of supply is 3 to 25 Hz
  • Electrode voltage is measured after magnetic field stabilization
  • Signal measurement period lengths is vital to eliminate noise

DC excited Application

  • Suitable for continuous flow
  • High accuracy & stability
  • Waveform automatically eliminate background noise

AC Magnet Coil Excitation

  • Coil is fed with 70 Hz AC current
  • Electrode voltage is measured continuously at all times

AC magnet coil excited Application

  • Fast batching or filling applications
  • Liquids with high content of solids e.g. mining slurries, paper pulp
  • Pulsating or rapidly changing flow applications.
DC and  AC Excitation of Electromagnetic Flowmeter

 

What Electromagnetic Flowmeter do?

         In water treatment facilities, magnetic flow meters are used to monitor water, chemicals, process water, and both treated and untreated sewage. Applications in the mining and mineral processing industries include heavy media flows, process water flows, and process slurry flows.

What are the difference between ultrasonic flow meter and electromagnetic flow meter?

         The versatility and level of accuracy offered by the electromagnetic flow meter are higher.

Why do we need to measure flow?

The quantity of fluid/ flow rate should be measure precisely for all process in industries and commercial purpose.

  • Process Variable Control
  • Efficiency
  • Safety
  • Custody Transfer
  • Monitor & reduce Wastage

What is the output of Electromagnetic Flow Meter?

The output of an electromagnetic flowmeter is typically an electrical signal that is proportional to the flow rate of the liquid. This signal can be in the form of a current or voltage, depending on the specific flowmeter.

The flowmeter will typically have a range of flow rates that it is capable of measuring, and the output signal will be within this range.

The output signal can be used to display the flow rate on a meter or to transmit the data to a remote location for monitoring or recording. Some flowmeters may also have additional output options, such as an analog output for connection to a control system or a digital output for communication with a computer or other device.

Real Time Applications of Electromagnetic Flow Meters

Electromagnetic flow meters are used in

  • Water management,
  • Processing industry,
  • life sciences,
  • Food industry,
  • In tunnel construction and mining, EMF are often the only way to accurately measure abrasive ore slurries, sand-water combinations, filler materials, or bulk solids,
  • In continuous flow (pumping water),
  • In Batch process (filling milk, or beverages) industries.

Compare characteristics of different types of flowmeters

ParameterElectro MagneticDifferential PressureCoriolisUltrasonic
Volume / Mass measurementVolumeVolumeMassVolume
Fluid/ Flow rateNot suitable for gas flowNot suitable for gases with flow rateNot suitable for very high flow ratesNot suitable for gas flow
Particulate flow / SlurriesSuitableConditionally suitableConditionally suitableConditionally suitable
Liquid/ Gas MixtureConditionally suitableNotsuitableConditionally suitableConditionally suitable
Liquid ConductivityOnly Conductive LiquidsSuitable for allSuitable for allSuitable for all
Food & Beverage
(Consumable liquids)
SuitableNot SuitableSuitableMost suitable for non-intrusive measurement
Installation / MaintenanceModern installation effort, minimal maintenanceEasy installation, Periodic maintenanceInstallation can be considerable, Relatively maintenance-freeEasy Installation and maintenance
Typical accuracy0.2 to 1% of reading0.6 to 2 % of full scale
0.1 to 0.5 % of reading

Doppler shift meter – 1% of reading to 2 % of full scale
Transit- time meter – 0.35% of reading to 2 % of full scale

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